Technical Papers

OLA TECHNICAL PAPERS is a special section of CLEAR VISIONS reserved for publishing technical articles of particular interest to those who own or operate optical laboratories. Concepts and opinions expressed in these monographs are those of the authors. The Optical Laboratories Association publishes these papers as a service to OLA members and assumes no responsibility for their content.

POLARIZED LENSES FOR CONTEMPORARY EYEWEAR

By Dr. Richard A. Phillips, President International Polarizer, Inc.*

Polarization is one of the fundamental properties of light waves. It was discovered in 1808 by E.L. Malus, a French army engineer. He was fascinated by the optical properties of the crystal calcite and frequently carried a piece with him to demonstrate its properties to his friends. One afternoon while looking through his crystal at the windows of the palace of Luxembourg, he noticed that the image changed as he rotated the crystal. He could not explain his observation but actually had discovered that light was polarized by reflection.

GLARE

When light is reflected from a surface, it is partially polarized. When it is reflected at a particular angle, 53 degrees in the case of water, it is completely polarized. Whenever such light - GLARE - is viewed through a polarizer, it is completely eliminated.

This glare produces eyestrain and discomfort. Tinted glass or plastic can not solve the problem of glare. Polarized filters selectively absorb the reflected glare while transmitting useful light from a scene. This overall transmittance can be adjusted to reduce brightness to a comfortable level.

Undesirable effects of glare are an increase in brightness and a reduction in color saturation. This makes objects viewed in the presence of glare appear washed-out.

AREAS WHERE GLARE IS IMPORTANT

WHEREVER THERE ARE HORIZONTAL SURFACES PRODUCING GLARE, THE USE OF POLARIZING LENSES IS INDICATED. It is widely recognized that around the water polarizing lenses are particularly effective. They are also useful around snow for skiers or hikers. Reflections from the roadway are partially polarized and drivers who wear polarized lenses will have improved vision. In sandy or desert areas vision will similarly be improved. In general, they are well suited for outdoor activities, which is precisely where sunglasses are used.

ULTRAVIOLET

Solar ultraviolet radiation is implicated in a number of eye conditions - pterygium (a growth on the conjunctiva), keratitis (accompanied by the sensation of sand in the eyes), and cataract (the yellowing and clouding of the lens over time). A number of epidemological studies have shown a relation between sunlight exposure and the development of cataracts. In the United States people in the southern areas have 4 times the number of cataracts as do the people in the northern areas. In laboratory experiments cataracts can be induced in animals by ultraviolet light and IT IS NOW WIDELY ACCEPTED THAT SOLAR ULTRAVIOLET LIGHT IS ONE RISK FACTOR IN CATARACT. In addition to eliminating glare, correctly designed polarizing lenses block out ultraviolet light. Ultraviolet absorbers can be incorporated at the manufacturing level by the producer of the lenses.

BLUE LIGHT

A number of laboratory studies have shown that light in the blue portion of the spectrum can create harmful photochemical changes on the retina. This has resulted in the incorporation of blue absorbing filters in lamps used in eye operations and the suspicion that blue sunlight is a contributing factor to macular degeneration. Much more data is needed before we can understand what is happening on the retine, but there is an increasing concern for the potentially harmful effect of blue light.

It can be easily demonstrated that blue absorbing lenses (yellow or amber) cut through atmospheric haze and in certain lighting improve contrast. By reducing the amount of blue light the effects of chromatic aberration of the lens is reduced. This leads to improved acuity. These physiological effects can be almost instantly perceived by someone who puts on a pair of blue attenuating lenses.

Polarizing lenses are available in two main versions: neutral gray which absorbs all colors approximately equally and blue absorbers which preferentially absorb blue light. However blue absorbers should not totally eliminate blue light because that would impede detection of blue objects.

CONCLUSIONS

Polarizing lenses have the unique ability to selectively eliminate glare. They have been used in over one billion pair of sunglasses over the last 50 years and their use remains widespread today because they have a clear functional benefit for the wearer. Modern versions block out ultraviolet light and offer the option of selectively attenuating blue light.

* Dr. Phillips received a Ph.D. in Physics from the University of Michigan. He held the position of Asst. Professor of Physics and Electrical Engineering at the University of Minnesota. He is co-author of "Contemporary Optics for Scientists and Engineers". He is President and founder of International Polarizer, Inc. of Marlboro, MA.

Richard A. Phillips, Ph.D.